Conveyor belts are used in a variety of industries (e.g., coal, aggregate, packaging) to transport goods and materials. Based on the material transported and the ambient conditions surrounding the conveyor belt, material can become undesirably affixed to the belt. The material can be removed from the belt in a number of ways, including, but not limited to the use of a belt cleaning apparatus utilizing spring tensioners.
A belt cleaning apparatus cleans the belt in place, during operation, via a scraping action thereagainst. A belt cleaning apparatus typically will include a cleaner blade or blades having a scraper or tip end that is biased into engagement with the belt surface, usually on the return run of the belt (secondary belt cleaners) or at the head pulley (primary belt cleaners). In either type of belt cleaner, the scraper blade will typically be mounted to an elongate pole extending below and across the belt to the frame structure of the conveyor system on either side of the belt for being operatively supported thereby. In this regard, one or both of the opposite ends of the pole can be mounted to biasing units secured to frame structures for providing a rotary bias force for urging the scraper blade into engagement with the belt surface.
Rotary spring tensioners generally include a collar secured to the elongate pole to which the belt scraper blades are attached. A lever arm extends from the collar and is configured to be connected to a spring device. Compression of a spring of the spring device creates a spring force vector which creates a moment about the elongate pole, thereby urging the scraper blades mounted on the pole into engagement with the conveyor belt.
The belt scraper blades of the belt cleaner are configured to wear over their useful life. One method of measuring the wear of the belt cleaner blades is a wear angle. The wear angle corresponds to the rotation of the elongate pole required to maintain tight engagement of the scraper blades against the conveyor belt. Scraper blades which are operable with a large wear blade angle are desirable as they have a longer useful life, which reduces the replacement frequency. Some prior art scraper blades have a useful life up to a wear blade angle of about 30 degrees.
As the scraper blades wear, the elongate pole rotates to maintain contact between the scraper blades and the conveyor belt. Rotation of the elongate pole results in a corresponding decompression of the spring, resulting in a lower spring force being applied to the lever arm. Over the useful life of the scraper blades, the spring tensioner must be retensioned to provide the appropriate moment about the elongate pole and thereby maintain engagement of the scraper blades with the conveyor belt. Retensioning includes recompressing the spring of the spring device to increase the spring force applied on the lever arm so that the cleaning blades are appropriately spring loaded into engagement with the belt.
Due to the rotation of the elongate pole, the position of the lever arm changes relative to the spring force vector. In turn, the efficiency of the transmission of the spring force to produce a moment about the elongate rod is affected. The efficiency of a rotary spring tensioner is measured by the transmission of force applied by the spring member into a moment about the elongate pole. In particular, the moment about the elongate pole is a function of M=lF sin Θ, wherein l is the lever arm length, F is the magnitude of the force applied by the spring, and Θ is the included angle between the axis of the lever arm and the vector of the force being applied on the lever arm by the spring. As such, the most efficient transmission of force is achieved when the angle between the lever arm and the force vector is 90°. As the angle increases or decreases from 90°, the spring force is less efficiently applied to the elongate pole secured by the collar.
Some prior art spring tensioners attempt to maintain the lever arm-spring force vector angle close to 90 degrees by varying the lever arm length during operation of the spring tensioner. An exemplary tensioner, such as the FLEXCO EST spring tensioner, includes a rod of the spring device extending from a fixed pivot on the mounting bracket. The rod extends through a closed slot extending along the lever arm. Mounted on the rod and engaging the lever arm is a spring. As the lever arm rotates due to blade wear, the rod shifts along the lever arm slot toward the distal end of the lever arm, thereby maintaining an angle of approximately 90 degrees between the lever arm axis and the spring force vector.
To accommodate the travel of the lever arm over the useful life of the scraper blade, the overall size of the rotary spring tensioner has to he configured to provide a lever arm slot long enough to maintain a spring force vector-lever arm axis angle of generally 90 degrees. As a result, the spring tensioner occupies a large volume during operation, in particular due to rotation of the lever arm and the movement of the spring rod along the lever arm slot to maintain a generally 90 degree angle between the lever arm axis and the spring force vector.
The space requirements of a spring tensioner are a function of the physical dimensions of the tensioner and the operational volume or envelope the spring tensioner occupies during operation. In particular, the operational volume or envelope includes the space occupied by both the lever arm and the spring assembly during operation. Some prior art spring tensioners reduce the operational volume by limiting the rotation of the collar and lever arm. More particularly, some prior art spring tensioners limit the angular rotation of the lever arm to an amount that may be less than the maximum useful wear angle of a particular scraper blade.
One known commercial tensioner is provided by ASGCO. The ASGCO Force-1 spring tensioner includes a collar having a lever arm extending away from the elongate pole. The ASGCO FORCE-1 spring tensioner includes spaced flanges of the mounting plate between which the lever arm extends and which limit the rotation of the lever arm. A spring rod extends from the lever arm and through an opening in one of the spaced flanges. The spring is mounted on the spring rod against the outer surface of the flange having the spring rod extending therethough.
The flanges of the ASGCO FORCE-1 spring tensioner are spaced to provide about a 22 degree rotation of the lever arm. By including two flanges, the mounting bracket allows for reconfiguration of the lever arm and spring device between providing a clockwise moment about the elongate pole and providing a counterclockwise moment about the elongate pole. In particular, the spring tensioner is reconfigured between providing clockwise and counterclockwise moments by removing the lever arm from the elongate pole, repositioning the lever arm and spring device to reverse their orientation, and then resecuring the lever arm to the elongate pole.
To minimize deviation of the angle between the lever arm axis and the spring force vector from about 90 degrees, the rod is not fixed to the flange of the mounting plate. Instead, as the lever arm rotates, the rod can translate and/or rotate to minimize deviation of the angle from about 90 degrees. However, even with limiting the movement of the lever arm, the spring tensioner still occupies a large operational volume or envelope due to the movement of the spring rod. In particular, as the elongate pole rotates due to blade wear, the lever arm shifts toward the spaced flange through which the spring rod extends. As the lever arm shifts closer to the flange, the spring rod translates relative to the mounting plate and extends further beyond the outer surface of the mounting plate flange. In addition, the spring rod rotates to minimize deviation of the angle from about 90 degrees, thereby increasing the operational envelope of the ASGCO FORCE-1 spring tensioner.
Further, as a result of limiting the rotation of the lever arm, a more complex retensioning process including repositioning of the lever arm may be required to accommodate the useful life of the scraper blades. In particular, once the lever arm can no longer rotate, the spring tensioner can not provide the necessary torque to the elongate pole to maintain the desired bias force on the scraper blades for engaging the conveyor belt. Therefore, an operator must decouple the collar from the elongate pole and rotate the collar to a position at or near the initial or starting position of the lever arm. The collar can later be repositioned again as necessary depending on the available lever arm rotation and the useful life of the scraper blade.